1. Field of the Invention
The present invention relates to an active composite having a laminate structure, consisting of a compressed support in the form of sheets and of an agent that is active with respect to a gas, the active agent being dispersed between the sheets in the form of granules.
The present invention also relates to a method of implementing gas-solid or gas-liquid physico-chemical processes using an active composite of this type as a reaction medium.
2. Description of Related Art
In certain fields such as, for example, that of chemical heat pumps based on the endothermicity or exothermicity of the reaction between a solid and a gas, or of the adsorption of a gas on a solid, a mixture of a divided material, such as expanded graphite, and of a solid reactant, for example a salt, or an adsorbent, such as a zeolite, is used. The mixture of expanded graphite and this solid, the site of a chemical reaction or of a physical adsorption, has many advantages during a chemical reaction or a physical adsorption between the solid and a gas. Expanded graphite, being in the form of sheets or of laminate particles, has a very large specific surface area and allows diffusion of the gas even in a confined medium.
The substantial improvement in the reversible solid-gas reaction kinetics which is observed, as resulting from the mixture of the active solid and natural graphite expanded by thermal means, in given mass proportions, and compacted in a fixed volume, results from an excellent permeability of the fixed bed prepared in this way to the reactive gas and from a thermal conductivity accompanied by a good coefficient of heat exchange with the walls. Conventionally, the preparation of such a reactant leads to an essentially homogeneous anisotropic bed whose anisotropic conductivity has values ranging from 0.5 to 2 Wm.sup.-1 K.sup.-1 depending on the conditions of preparation (proportion and compacting) and whose coefficient of exchange with the walls lies between 50 and 150 Wm.sup.-2 K.sup.-1.
Despite these advantages, the use of such mixtures in the granular state has drawbacks because of the difficulty involved in obtaining genuinely homogeneous mixtures, because of the difficulty involved in handling them and because of the large volume they occupy. In addition, the reactant, which is often hygroscopic, tends to absorb moisture if the mixing operation is a long one; a subsequent dehydration step is lengthy, and indeed expensive, and may affect the quality of the end-product, even after dehydration.
Document WO91/15292 describes an active composite in the form of a block, which comprises recompressed expanded graphite, which is subsequently impregnated with an active agent, for example a salt. This type of active composite has major advantages compared with the powder mixtures containing expanded graphite which are described above, but it may nevertheless be difficult to produce since, when the block is impregnated with a liquid, it may take a long time to dry out the block.
The methods of manufacturing expanded graphite are well known, in particular from U.S. Pat. No. 3,404,061. These methods, called graphite exfoliation methods, include a step in which a graphitic complex is rapidly expanded by heating, producing a powder of vermicular expanded graphite.
Document FR-A-2,732,242 describes an active composite formed by a series of compressed-graphite sheets superposed on top of one another, an active agent being dispersed within the sheets. Typically, during the manufacture of this type of composite, the sheets are immersed in a solution of the active agent, then dried afterwards.
Document FR-A-2,715,081 discloses a reactant, for thermochemical systems, in the form of granules. Each granule comprises a support formed by recompressed expanded natural graphite which is impregnated with an active agent. The impregnation with the active agent takes place by immersing the granules in a solution of the agent, then by drying them afterwards.
Since the method of manufacturing the two previous types of reactant includes an impregnation step followed by a drying step, it may prove to be long and expensive.